1. Field of the Invention
The present invention relates to an optical amplifier which can amplify a plurality of channels of signal light having wavelengths different from each other.
2. Related Background Art
In a photonic network 100 shown in FIG. 1, optical ADMs 101 to 104 and optical XCs are arranged on a network including a plurality of optical amplifiers 111 to 114, whereby the number of channels of signal light inputted into the optical amplifiers 111 to 114 and the power of these channels may fluctuate. One of important performances required for such an optical amplifier is a transient response characteristic in high-speed automatic gain control (AGC). When signal light whose power is fluctuated by an optical ADM is inputted into an optical amplifier as shown in FIG. 1, the gain of signal light remaining in an optical amplifier medium may fluctuate within the optical amplifier.
When the input signal light power decreases, a phenomenon (overshoot) in which the gain fluctuation of signal light consistently remaining in the optical amplifier medium transiently increases too much occurs in general. When the input signal power increases, on the other hand, a phenomenon (undershoot) in which the gain fluctuation of signal light consistently remaining in the optical amplifier medium transiently decreases too much occurs. Both the overshoot and undershoot have been known to adversely affect optical transmission systems. The overshoot may cause an excessive power input to make receivers out of order. The undershoot may deteriorate the power on the outside of a receivable range, thereby causing transmission errors. These make it necessary to suppress the over/undershoot in order to realize a high-quality optical transmission system.
Known as an optical amplifier widely utilized in known optical transmission systems is a structure in which an optical fiber having an optical waveguide region doped with a rare-earth element is included as an optical amplifier medium. In particular, optical amplifiers (EDFAs: Erbium-Doped Fiber Amplifiers) including an optical fiber (EDF) doped with erbium as a rare-earth element have been widely used. For suppressing the over/undershoot, conventional EDFAs have monitored input/output signal powers and regulated the pumping light power at a high speed so as to keep a constant amplification gain. In the optical amplifiers disclosed in Patent Documents 1 to 5, rare-earth-element-doped optical fibers whose cutoff wavelength is set longer than the pumping light wavelength but shorter than the signal light wavelength in order to suppress the over/undershoot are employed as optical amplifier media. Patent Document 6 discloses a technique in which a fiber propagating signal light is bent, so as to eliminate signal light components of modes higher than the fundamental mode.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-168427
Patent Document 2: Japanese Patent Application Laid-Open No. 2001-127365
Patent Document 3: Japanese Patent Application Laid-Open No. HEI 10-012952
Patent Document 4: Japanese Patent Application Laid-Open No. HEI 10-270785
Patent Document 5: Japanese Patent Application Laid-Open No. HEI 10-303490
Patent Document 6: Japanese Patent Application Laid-Open No. HEI 4-298724